Abstract: We present a theoretical study of the reaction of the hydroxyl radical with ethene using electronic structure calculations and direct-dynamics simulations. High-accuracy electronic structure calculations at the CCSD(T)/aug-cc-pVTZ//MP2/aug-cc-pVDZ level have been carried out to characterize the representative regions of the potential energy surface of various reaction pathways, including OH-addition and H-abstraction. These ab initio calculations have been employed to derive an improved set of parameters for the MSINDO semiempirical Hamiltonian specific to the OH+C₂H₄ reaction. The specific-reaction-parameter Hamilto-nian captures the ab initio data accurately, and has been used to perform direct quasiclas-sical trajectory simulations of the OH+C₂H₄ reaction at collision energies in the range of 2?10 kcal/mol. The calculated cross sections reveal that the OH-addition reaction domi-nates at all energies over H-abstraction. In addition, the excitation function of addition is reminiscent of a barrierless capture process, while that for abstraction corresponds to an activated one, and these trends can be connected to the transition-state energies of both reactions. We note that the development of an accurate semiempirical Hamiltonian for the OH+C₂H₄ reaction in this work required the inclusion of empirical dispersion corrections, which will be important in future applications for which long-range intermolecular attraction becomes significant